Investigators from the National Transportation Safety Board (NTSB) have confirmed that engineers were adjusting the tension on two tensioning bars in the 53m long post-tensioned bridge’s concrete truss when it collapsed.

NTSB officials confirmed that the two tensioning bars being worked on were in a diagonal member at the north end of the span – the location where the collapse was triggered. Tension had already been applied to one bar and engineers were working on the second bar at the north end when the span failed and collapsed.

NTSB said the same work had already been carried out earlier at the south end.

The work on the bridge was being carried out over a live, eight lane highway when it collapsed, killing six people.

Investigators said they were now focusing on measuring and documenting the critical structures at the north end of the bridge.

Significant developments in the investigation include the securing of a contract to remove components from the bridge to undergo examination and testing, including sections of the floor, the canopy, a vertical member and a diagonal member; all from the north end of the structure.

Concrete core samples from the failed area have also been extracted and sent to a facility for testing and evaluation along with recovered rebar and tensioning bars.

NTSB said while segments of the bridge were being transported to and stored at a Florida Department of Transportation facility, there was no plan to reconstruct the bridge as part of the collapse investigation.

“The nature of the structure and the way it failed make reconstruction impractical,” said the NTSB.

It emerged last week that engineers had observed, discussed at length and ultimately dismissed the significance of a cracking in the pedestrian bridge just three hours before it collapsed. Investigators have not yet released details of the location of the cracking.

Readers' comments (2)

Carrying out post-tensioning operations over a live road is okay, so long as the deck is supported on formwork, etc. What makes it criminally negligent, in this case, is the lack of redundancy if something goes wrong. Didn't anybody at the DoT sit back and say whoa? Obviously not.

Also, after further research, I'm amazed to find the so called 'stays' which were to be added in the final stage, were fake, insofar as they were simply 16" dia. steel tubes, bolted to the deck and pylon. I don't know if they were intended to carry any significant loads, but apparently their main purpose was to dampen the natural frequency of the deck and look pretty.

It's also interesting the see the diagonal at the other (south) end appears to be considerably deeper that that at the north. Yes, the angle is a bit shallower, but not that much, so it's carrying more or less the same load. It doesn't make much sense.

In fact the whole bridge doesn't make much structural sense. It's a case where engineering seems to have taken a back seat to aesthetics. Reminds me of a bridge proposed a while ago in this country, but was thankfully never built. Wear on earth could that have been, I wonder.

The failure would seem likely to me to have propogated at the bottom end of the first diagonal member. From the photos of the cracking pre-collapse, the diagonal appears to be punching through the end of the deck, but is being partially held by the end vertical member. The loss of prestress in the diagonal, which was apparently being restressed when collapse occurred, seems to be an indication that the diagonal is beginning to punch its way through the deck and be locally damaged. The "punching" behaviour is exacerbated in this bridge by the form of design whereby there is only a single central arrangement of diagonal members transmitting forces into a wide slab, such that "shear lag" is severe where the diagonals intersect the deck. The prestressing of the deck is distributed over its full width and would not be able to deal with the very localised compression force from the diagonal at the end of the bridge.
Thus I would tend to suspect a design flaw. The prestressing of an end compression diagonal was presumably to overcome the temporary condition during erection when the bridge was being supported inboard from its ends.The restressing being attempted once the bridge was permanently supported at its ends was probably the "straw that broke the camel's back". The design had been further compromised by the unusually flat angle of the diagonals to match with the "dummy" cable staying of the bridge. There is surely a lesson here in "imitation architectural concepts", whereas bridges should be about honest structural form.
Alan Hayward FREng CEng FICE FIStructE

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